Solid state light sources, such as but not limited to light emitting diodes (LEDs) and other semiconductor-based light sources, require a constant direct current (DC) voltage or current to operate optimally. During operation, the light source must be protected from line-voltage fluctuations. Changes in voltage can produce a disproportional change in current, which in turn can cause light output to vary, as solid state light source light output is proportional to current and is rated for a current range. If current exceeds the manufacturer recommendations, the output of the solid state light sources can become brighter, but that output can degrade at a faster rate due to higher temperatures within the device, which leads to a shorter useful life.
Solid state light sources, therefore, require a driver (also referred to as a driver circuit and/or a power supply) that converts incoming alternating current (AC) power to the proper DC voltage, and regulates the current flowing through the light sources during operation. The driver converts 120V (or other voltages) 60 Hz AC power to DC power required by the light sources, and protects the light sources from line-voltage fluctuations.
Drivers can enable dimming of solid state light sources via, for example, preset commands, occupant presence, or other controls. Drivers with dimming capability can dim the light output over the full range from one hundred percent to zero percent. Dimming drivers can dim solid state light sources by, for example, reducing the forward current, pulse width modulation (PWM) via digital control, or more sophisticated methods. Most dimming drivers operate using the PWM method. With this method, the frequency could range from a hundred modulations per second to as high as hundreds of thousands of modulations per second, so that the solid state light source appears to a human eye to be continuously lighted without flicker. A benefit of the PWM method of dimming is that it enables dimming with minimal color shift in the light source output. Dimming does not result in a loss of efficiency. During dimming, the solid state light sources are still operated at the same voltage and current as during full light output. In addition, life of a lighting device including the solid state light sources is not affected by dimming, as is sometimes the case with frequently dimmed fluorescent lamps. Rather, dimming solid state light sources may lengthen their useful life, because dimming can reduce operating temperatures inside the light source.